The measure, now set for a floor vote Tuesday, would give states more flexibility in how they spend federal money and would step up the pace of road construction by shortening environmental reviews. Even the measure's sponsors—California Democrat Barbara Boxer and Oklahoma Republican James Inhofe—come from opposite political poles. Passage would provide a rare display of bipartisan deal-making between the parties in a bitterly partisan election year.

We are hopeful this will become a template for all of us in the Senate and the House to find the sweet spot where we can work together," Boxer said. She said the Senate's bipartisan success should be a lesson for House Republican leaders, whose efforts to pass their own bill without concessions to Democrats have fallen apart.

Nearly 20 amendments—including proposals to put toll booths on interstate highways, provide tax credits for alternative energy, and tighten requirements that highways and bridges be constructed using domestic goods and labor —were slated to be voted on in an all-day marathon of Senate voting.

The first—an amendment that would have allowed states to keep nearly all of the federal gas taxes collected within their borders, thus depriving federal highway and transit programs of most of their funds—was soundly rejected.

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The overall bill would spend $109 billion over less than two years. That's far below the level of spending that two congressional commissions have said will be needed if the U.S. is to maintain its aging roads and bridges and bus and train systems and expand the national transportation network to meet population growth between now and 2050. Transportation Secretary Ray LaHood has described the nation's roads as "one big pothole."

Senate Democrats have been pitching the measure as a jobs bill, predicting it would preserve or create nearly 3 million jobs. But economists say it's unlikely the bill would create any more jobs than would be created by the same amount of money spent on something else.

The bill would increase the flow of highway aid to states by adjusting current spending levels to take into account inflation over the past several years. States would have greater discretion over how to spend the money, but the bill also would create a new regime of performance and project eligibility requirements aimed at preventing waste and making sure national goals are met.

A credit assistance program that helps leverage private investment for transportation projects of national and regional significance would be increased tenfold to $1 billion. In the past, the program has been able to generate as much as $30 in private capital for every $1 in aid, Boxer has said.

The measure also would reduce the number of federal transportation programs by roughly two-thirds in an effort to eliminate duplication. Bicycle, pedestrian, safe routes to schools and rails-to-trails programs—which were targeted by Republicans—were preserved by moving them into a larger congestion mitigation program where they would have to compete with other programs for money.

Safety measures in the bill would toughen regulation of the long-distance and tour bus industries, including setting deadlines for government requirements that buses have seat belts, improved roof strength, anti-ejection window glazing and rollover crash avoidance systems. The bus industry transports about 750 million passengers a year, roughly the same as the domestic airline industry.

One thing the bill doesn't do is resolve how to keep the Federal Highway Trust Fund solvent beyond next year. The fund pays for highway and transit programs, but revenues have declined while spending has remained the same. The largest source of money for the fund is federal fuel taxes: 18.4 cents a gallon for gasoline and 24.4 cents a gallon for diesel. Revenue from those taxes has been lower since the economic downturn in 2008 and because the fuel efficiency of cars and trucks is increasing.

The bill would pay for highway programs through a combination of fuel taxes, cuts to other federal programs and tax changes, but also would drain the trust fund. Some senators have been critical of the provisions that are supposed to pay for transportation programs since they would raise about $10 billion over 10 years, but spend it in the first two years.

The highway bill "is so popular that members on both sides of the aisle are willing to kick the can down the road," Sen. Bob Corker, R-Tenn., said. "I think the American people understand that passing a bill that spends money over two years and tries to recoup it over a 10-year period is a highway to insolvency."

Lawmakers are under pressure to act quickly. The government's authority to raise money through fuel taxes and spend money from the trust fund expires March 31. The fu el taxes raise about $110 million a day for the government.

In the House, Speaker John Boehner, R-Ohio, made a five-year transportation bill the election-year centerpiece of the GOP's jobs agenda last fall when he unveiled its broad outlines. Unable to corral enough votes for passage of the GOP measure, Boehner said last week that he plans to bring the Senate bill to the House floor for a vote. But that could change when the House returns from a weeklong recess next week if GOP leaders are able to snare enough votes for their own measure.